Cellular radio systems are increasing in use throughout the world providing telecommunications to mobile users. In order to meet with capacity demand, within the available frequency band allocation, cellular radio systems divide a geographic area to be covered into cells. At the centre of each cell is a base station, through which the mobile stations communicate. The available communication channels are divided between the cells such that the same group of channels are reused by certain cells. The distance between the reused cells is planned such that co-channel interference is maintained at a tolerable level.
When a cellular radio system is initially deployed, operators are often interested in maximuzing the uplink (mobile station to base station) and downlink (base station to mobile station) range. The range in many systems are uplink limited due to the relatively low transmitted power levels of hand portable mobile stations. Any increase in range means that less cells are required to cover a given geographical area, hence reducing the number of base stations and associated infrastructure costs. Similarly, when a cellular radio system is mature the capacity demand can often increase, especially in cities, to a point where more smaller size cells are needed in order to meet the required capacity per unit area. Any technique which can provide additional capacity without the need for cell-splitting will again reduce the number of base station sites and associated infrastructure costs.
The sectorised approach to the use of directive antennas has reached its useful limit at 60.degree. beamwidth and can go no further. The key disadvantages of this sectorised approach are: the cellular radio transceivers are dedicated to particular sectors which leads to significant levels of trunking inefficiency. In practice this means that many more transceivers are needed at the base station site than for an omni-directional cell of the same capacity, and; each sector is treated by the cellular radio network (i.e. the base station controller and mobile switches) as a separate cell. This means that as the mobile moves between sectors, a considerable interaction is required between the base station and the network to hand off the call between sectors of the same base station. This interaction, comprising signalling and processing at the base station controller and switch, represents a high overhead on the network and reduces capacity.
The antenna used at the base station site can potentially make significant improvements to the range and capacity of a cellular radio system. The ideal base station antenna pattern is a beam of narrow angular width. The narrow beam is directed at the wanted mobile, is narrow in both the azimuth and elevation planes, and tracks the mobiles movements. Within current systems the manner in which directive antennas are used allows relatively small benefits to be obtained. The use of directive antennas, however, in current cellular radio systems, is based on the principle of sectorisation.
U.S. Pat. No. 4,128,740 (Graziano) is typical of many descriptions of cellular communication systems: an array of antennas is provided at each cell site for providing communications to randomly placed transceivers in a given area. Each antenna site has a plurality of sectored antennas for providing a plurality of communication channels. A predetermined number of sites are used to constitute a sub-array of cells to provide a set of communication channels and channel allocations are repeated from subarray to subarray. Channels are allocated per sub-cell so as to minimize channel interference. Each antenna thus is required to subtend an arc of, typically 60.degree. or 120.degree., depending on the number of antenna arrays employed. Accordingly the transmit and receive electronics must be sufficiently powerful to cope with transmitting and receiving over a wide arc. Such transmit and receive electronics, including the amplifiers are situated at the bottom of the antenna structure.
Multiple narrow beams can be formed in several distinct ways, depending on the structure used to form the basic narrow beam. This can be (a) a reflector, (b) a lens or c a phased array antenna. For (a) or (b), an array of feeds is used, with the reflector or lens forming a three-dimensional structure. For (c) a planar structure can be used, and this is highly desirable for a cellular base station, where low profile and low windage are key attributes.
U.S. Pat. No. 4,626,858 (Copeland) provides a system for receiving signals from airborne objects such as telemetry data transmitted during the terminal phase of a re-entry ballistic missile, comprising an array fed aperture, with a Luneberg lens array fed aperture antenna being described. Receive amplifiers only are situated behind the multiple feeds. A large volume is required for the lends, unlike a phased array multiple beam antenna.
With a phased array multiple beam former, transmit and receive amplifiers can be associated with each column of the array. In conventional systems the amplifiers tends to be mounted as discrete components since such amplifiers and associated electronics are liable to fail and (the power amplifiers are the most unreliable part of a cellular site) accordingly a re located in an electronics control cabinet at the base of a mast or building which supports the antennas. If a system fails, then access for repair and the like is relatively straightforward. Typically the power of the transmit amplifiers employed in phased array telecommunications antennas is around 40 watts to cope with transmission losses which occur as signals are sent up the antenna mast or building, from the base station control electronics to the antennas at the masthead. The r.f. feeder cables must be very low loss and become large and expensive.